1. Field of the Invention
Aspects of the present invention are directed to the processing of an Orthogonal Frequency-Division Multiplexed signal, and more particularly to a system and method for determining transmission parameters in an Orthogonal Frequency-Division Multiplexed signal in which the determined transmission parameters may additionally be used to determine a coarse FFT window position based thereon.
2. Discussion of the Related Art
In Orthogonal Frequency-Division Multiplexing (OFDM) systems, information such as compressed audio and/or video data is carried via a large number of individual carriers (i.e., sub-carriers) in a frequency multiplex. The frequencies of the sub-carriers are selected so that the modulated data streams are orthogonal to each other, thereby eliminating cross-talk issues. Although each carrier transports only a relatively small amount of information, high data rates may be achieved by using a large number of carriers (e.g., 2048, 4096, 8192, respectively termed 2k, 4k, and 8k mode) multiplexed together. The individual carriers are modulated (e.g., using phase-shift keying (PSK) techniques, or amplitude modulation techniques, such as Quadrature Amplitude Modulation (QAM)), with each carrier having a fixed phase and amplitude for a certain time duration, during which a small portion of the information is carried. That small portion of information is called a symbol, and the time period for which it lasts is called the symbol duration. The modulation is then changed and the next symbol carries the next portion of information. Examples of known OFDM systems include DVB-T (Digital Video Broadcasting-Terrestrial) Standard systems, DVB-H (Digital Video Broadcasting-Handheld) Standard systems, T-DAB (Terrestrial-Digital Audio Broadcasting) Standard systems, 3G and 4G mobile phone wireless network systems, as well as others.
In OFDM systems, modulation and demodulation are performed using the Inverse Fast Fourier Transformation (IFFT) and the Fast Fourier Transformation (FFT), respectively. The time duration of a symbol is the inverse of the carrier frequency spacing, thereby ensuring orthogonality between the carriers. In addition to the data that is carried by an OFDM signal, additional signals, termed ‘pilot signals’ (whose value and position are defined in the applicable standard, and are thus known by the receiver) are inserted into each block of data for measurement of channel conditions and also for synchronization.
In order to overcome inter-symbol interference, a portion of each symbol (e.g., the first portion or the last portion) is copied and appended to the beginning or end of the symbol. For example, in DVB-T standard systems, the last portion of the symbol is copied and appended to the beginning of the symbol as a cyclic prefix. In OFDM systems, and as used herein, that copied portion of the symbol is termed the “guard interval” and its duration (or length) is typically denoted L, the duration of the original symbol (i.e., the “useful symbol duration”) is typically denoted TU, and the increased symbol duration is typically denoted TS, where TS=TU+L. Provided that most (or ideally all) echo energy from a prior symbol falls within the guard interval, the symbol may still be recovered.
In an OFDM receiver, the received OFDM signal is demodulated to baseband using some type of quadrature amplitude demodulation or phase shift keying demodulation, the resultant baseband signals are then typically low-pass filtered, and the filtered baseband signals are then sampled and digitized using analog to digital converters (ADCs). After removal of the guard interval, the digitized signals are then provided to an FFT module and converted back to the frequency domain. Because of the presence of the guard interval, a nearly infinite number of options exist as to where to place the FFT window to evaluate the symbol. In general, it is desired to place the FFT window on the useful part of the symbol (TU), and so that all or nearly all echo energy lies within the guard interval (L) of the symbol.
One system and method for determining the placement of an FFT window on the useful part of a symbol (TU) so that all, or nearly all, echo energy lies within the guard interval (L) of the symbol is described in commonly owned U.S. patent application Ser. No. 11/731,791, filed on Mar. 30, 2007 by Itamar Gold-Gavrieli et al. and entitled SYSTEM AND METHOD FOR FFT WINDOW TIMING SYNCHRONIZATION FOR AN ORTHOGONAL FREQUENCY-DIVISION MULTIPLEXED DATA STREAM (hereinafter “the co-pending application”), which is incorporated herein by reference in its entirety. Although the system and method described in the co-pending application may be used to accurately determine an optimal placement of the FFT window, this ability is premised on the system's knowledge of the transmission mode (e.g., 2K mode, 4K mode, 8K mode) and the guard interval of the OFDM signal.